FIG. 11 shows an example of the structure of a conventional power semiconductor device investigated by the inventors of the present invention as a premise.
The conventional power semiconductor device 110 comprises a power semiconductor element 111 die-bonded on a frame 112 with a solder 117. After bonding the element to a lead 113 with a wire 114, the semiconductor device is molded with an epoxy-based resin 115. A high-lead solder or a solder containing a trace amount of Ag and Cu and having a melting point of 290° C. or higher is used as the solder 117 (the term melting point in the present invention refers to solidus temperature).
The maximum temperature sometimes reaches 280° C. in the wire-bonding process. In addition, the melting points of those Sn—Ag—Cu-based lead-free solders mainly used hereinafter are as high as about 220° C., and the semiconductor device is supposed to be heated at 260° C. in maximum for reflow bonding. Accordingly, a solder having a melting point of higher than 280° C., or the above-mentioned high-lead solder is used so that the solder 117 does not re-melt in the wire bonding process and reflow process.
While the joint between the power semiconductor element 111 and frame 112 bonded with the solder 117 is molded with the epoxy-based resin 115, the solder 117 within the semiconductor device may leak out of the interface between the epoxy-based resin 115 and frame 112 due to cubical expansion by melting as shown in FIG. 12. This phenomenon is called flush. Or, even when the solder does not leak, it acts as if it wished to leak. Consequently, large voids 118 are formed within the solder after solidification to render the semiconductor device defective.
The significance of the solder 117 part at the joint is not only to fix the power semiconductor device 111 on the frame 112, but also functions to release the heat of the power semiconductor element 111 to the frame 112 side. Accordingly, when the voids 118 are formed by re-melting of the solder 117, heat dissipation through the joint becomes insufficient, and the function of the power semiconductor element 111 may be deteriorated.
In accordance with enforcement of the EU RoHS instruction (regulation of use of harmful substances used in electric and electronic appliances) on Jul. 1, 2006, developments of the lead-free solder for bonding a semiconductor element to a substrate has been rapidly progressed mainly on Sn—Ag—Cu-based lead-free solders.
On the other hand, die-bonding using the high-lead solder has been excluded out of the above-mentioned regulation because no technical solutions on the substitute of this lead-free solder have been invented. However, since the content of lead in this solder is as high as 90% or more, elimination of lead from the solder is desirable from the viewpoint of reduction in the environmental load.
However, reflow soldering to the substrate using the Sn—Ag—Cu-based lead-free solder is an inevitable step in the method for using the lead-free solder at the die-bonding joint. Accordingly, such lead-free solder should have a melting point of at least 260° C. or higher.
Sn—Sb-based solders (melting point: 232 to 240° C.) are examples of the Sn-based lead-free solders having a relatively high melting point. However, the melting point of these solders is yet so low that they cannot be used in the post process involving the re-melting process.
While Au—20Sn solder (melting point: 280° C.) is well known as lead-free high melting point solder, applications of this solder to cheap electronic parts are difficult when the cost is considered since the solder is expensive due to high content of Au of as much as 80%. Since this solder is a hard solder, the stress-buffering ability of this solder is insufficient for applying it to die-bonding that requires a relatively large bonding area. Therefore, the semiconductor element or joints thereof may be broken when the device is supposed to suffer from repeated thermal fatigue, and reliability of bonding may be impaired.
While the problem of reliability may be improved by increasing the amount of the supplied solder, this also causes another problem of profitability since the manufacturing cost further increases as the amount of supply of the solder increases.
Williams, W. et al. reported that the solder is able to have a high melting point by alloying the joint when the lead-free solder is used at the joint (Williams, W. et al., High Temperature Joints Manufactured at Low Temperature, Proceeding of ECTC., 1998; referred as non-patent document 1 hereinafter). The document reports that the joint is substantially converted into a Cu3Sn compound in order to permit the joint to have a high melting point by holding the joint for 16 hours after bonding GaAs metallized with Cr (0.03 μm)/Sn (2.5 μm)/Cu (0.1 μm) on the back surface to a substrate (a glass) metallized with Cr (0.03 μm)/Cu (4.4 μm)/Au (0.1 μm) at 280° C.
It is also reported that the joint can be made to have a high melting point by converting the joint into an Ag-rich alloy+Ag3In intermetallic compound by treating the joint at 150° C. for 24 hours, after bonding Si metallized with Cr (0.03 μm)/In (3.0 μm)/Ag (0.5 μm) on the back surface to Si metallized with Cr (0.03 μm)/Au (0.05 μm)/Ag (5.5 μm)/Au (0.05 μm) at 210° C.
Yamamoto et al. reported that all the joints are able to have high melting points by converting the joints into an intermetallic compound comprising Ni3Sn4, (Ni, Co)Sn2+(Au, Ni, Co)Sn4 by bonding Ni-xCO (X=0.10) metallized with Sn—3.5 Ag (26 μm) to Kovar metallized with Ni—20Co (5 μm), on which Au (1 μm) is further metallized, at 240° C., and by holding the joint for 30 minutes at that temperature (Yamamoto et al., Study on Intermetallic Compounds at Micro-Joint Using Sn—Ag Solder, Abstract of MES 2003, October, 2004, p45; referred as non-patent document 2 hereinafter). This means that using Ni—20Co containing Co for metallizing permits the growth rate of the compound to be enhanced.
When the joint has been once made to have a high melting point by the above mentioned method, the joint can be maintained without being re-melted even by heating at 260° C. in the reflow soldering process.